Change of emitted optical wavelength versus time, e.g. wavelength-sweep, is used within many photonic systems, such as during the interrogation of fiber-optic sensors, various spectrometric applications including chemical, biochemical and bio-medical sensing, telecommunication component measurements and testing, and the like. Furthermore, rapid wavelength sweep-rates are often desired. Rapid wavelength sweep allows for high-speed interrogation of optical sensors, which is often desired in dynamic sensing applications. Within multiplexed fiber-optic sensor arrays fast wavelength sweeps provide an opportunity for the time division multiplexing of sensors or sensor groups within sensor arrays. Multiplexed sensor arrays often consist of many spectrally-resolved sensors, such as fiber Bragg gratings, positioned along a single fiber. Interrogation of such arrays often requires high power optical sources to compensate for optical losses that are present in such arrays. In order to apply time-division multiplexing in such arrays, the wavelength sweep should be typically completed in less than about 1 μs, more typically in less than a few hundreds of ns, as the duration of the sweep determines the minimum distance amongst individual sensors with the same wavelength characteristics.
While there are many known solutions within laser design that can provide wavelength sweeping capability, the sweeping of wavelengths associated with semiconductor laser diodes remain limited to several relatively-complex solutions. Traditionally, semiconductor tunable/sweepable laser sources incorporate a semiconductor gain medium (e.g., a laser chip) and an external wavelength tunable optical feedback. One example is a large external cavity laser diode as described in U.S. Pat. No. 7,212,560. Such tunable/sweepable semiconductor lasers are usually configured within a Littman-Metcalf cavity arrangement. In this configuration, a laser chip which provides optical gain is coupled to external mode-selection filtering and tuning elements via bulk optical elements. In most external-cavity approaches, the external mode-selection filter is a diffraction grating that can also double as a mirror. External optical elements are mechanically adjustable to allow for changes in optical feedback geometry, which further leads to a change in the emitted wavelength. Such a configuration requires moving micro-mechanical parts that need to be configured into a well-defined geometrical configuration. Such tunable sources are therefore complex and expensive for production. Moreover, the sweep-rate may be limited by the mechanical system's Eigen frequencies. Thus, their wavelength sweep-rate may seldom exceed a kHz repetition rate.
Several types of fully-integrated tunable laser diodes have also been successfully developed. Such laser diodes do not require any external mechanical parts, since wavelength tuning is achieved directly by structures integrated into a laser chip. The first group of tunable laser diode sources utilizes distributed back reflectors (DBR) located at the side or sides of the active region of the laser diode. This provides wavelength selective optical feedback necessary for diode lasing. DBR diodes designs includes a two-section DBR (distributed back reflector) laser diode described in U.S. Pat. No. 6,862,394, a three-section DBR laser diode described in U.S. Pat. No. 6,806,114, and a multi-section DFB laser diode described in US Publication No. 2004/0136415. Wavelength tuning of such laser diodes is based on changes in the refractive indexes of one (two-section) or two (three-section) DBR regions of the laser diode. Change in the refractive index of a DBR region causes a shift in spectral characteristics of a laser diode's optical feedback, which results in a change of the output wavelength. Changes in the refractive index of the described laser diodes may be caused by electrical current, but can also be achieved by local changes of temperature, with heat-strips located at specific regions, as described in US Publication No. 2010/0309937. While DBR diodes can provide fast wavelength sweeping, they are relatively complex for production, may exhibit mode-hopping, and may be prone to generating additional noise. Generally, due to their complex structure, they are not widely used.
The second group of tunable laser diodes utilizes wavelength tuning structures that are integrated along (i.e. in parallel with) the diode's active region. One example is a tunable twin guide DFB (TTG-DFB) as described in U.S. Pat. No. 7,112,827. Another is a striped heater DFB laser as described in U.S. Pat. No. 5,173,909. This group of tunable laser diodes allows for wavelength tunability without mode hopping effects. Direct heating can typically induce wavelength sweeps within a range of about 5-10 ms.
Most existing tunable laser diode systems, as described above, have complex structures and are consequently not produced in high volumes. The tuning range of such laser sources do not often exceed about 8 nm and the diodes may exhibit mode-hopping effects. Except in the case of DBR-based diodes, the tuning speed/rate is usually also limited. This especially applies to all systems that utilize temperature-induced effects for wavelength tuning.
Due to the relatively high complexity and high costs of known tunable laser diodes and methods, there is a need for simpler and more cost-effective tunable laser sources. This is especially true in the field of interrogation of optical sensor(s). Thus, improved methods and systems to produce wide and rapid wavelength sweeps are sought.